Loft Conversion Cost UK 2026: Prices by Type and Size
What a loft conversion really costs in 2026, by type and size, with the fees, extras and building regulations that shape the final figure, a worked example and how long each type takes.
Key takeaways
- A rooflight loft conversion typically costs £22,000 to £40,000 nationally in 2026; a rear dormer £40,000 to £65,000.
- Hip-to-gable conversions cost £50,000 to £75,000 and mansards £60,000 to £90,000 or more.
- Many lofts are permitted development: 40 m³ on a terrace, 50 m³ on a semi or detached house.
- Building regulations always apply, including a protected escape route and fire doors on the stairs.
On this page (16 sections)
- Loft conversion cost at a glance (2026)
- Loft conversion cost per m²
- The four types of loft conversion
- What does a loft conversion cost, line by line?
- Loft conversion cost by house type
- What fees do you pay on top of the build?
- Do you need planning permission for a loft conversion?
- What building regulations apply to a loft conversion?
- Worked example: a rear dormer on a Southampton terrace
- The loft conversion process, step by step
- How long does a loft conversion take?
- What head height do you need for a loft conversion?
- How to keep the cost down
- Mistakes that push the cost up
- Questions to ask a loft conversion company
- Loft conversions in Southampton and Portsmouth
A loft conversion in the UK typically costs £22,000 to £40,000 for a rooflight (Velux) conversion, £40,000 to £65,000 for a rear dormer, £50,000 to £75,000 for a hip-to-gable and £60,000 to £90,000 or more for a mansard in 2026, excluding VAT. In Southampton and the rest of south Hampshire, budget roughly 10 per cent above those national figures.
The type of conversion sets the price band, and three things move you within it: whether you add an en-suite, where the new staircase lands, and whether your roof is a traditional cut roof or modern trussed rafters. This guide breaks each of those down, with the fees, building regulations and planning rules behind the build, a line-by-line worked example and the questions to ask before you sign anything.
Loft conversion cost at a glance (2026)
| Loft conversion type | National typical cost | South Hampshire typical cost | Typical time on site |
|---|---|---|---|
| Rooflight (Velux) | £22,000 – £40,000 | £25,000 – £45,000 | 6 – 8 weeks |
| Rear dormer | £40,000 – £65,000 | £45,000 – £70,000 | 8 – 12 weeks |
| Hip-to-gable | £50,000 – £75,000 | £55,000 – £80,000 | 10 – 14 weeks |
| Mansard | £60,000 – £90,000+ | £65,000 – £95,000+ | 10 – 14 weeks |
Loft conversion cost by type, south Hampshire (2026, ex VAT)
Loft conversion cost per m²
As a rule of thumb, a complete loft conversion costs roughly £900 to £3,000 per m² of new floor space, from a simple rooflight conversion at the bottom to a mansard at the top. Small conversions sit higher in the range because the fixed costs, the staircase, scaffolding, fire safety and fees, are spread over fewer square metres.
| New floor area | Rooflight | Rear dormer | Hip-to-gable |
|---|---|---|---|
| 20 m² | £22,000 – £32,000 | £40,000 – £50,000 | £50,000 – £60,000 |
| 30 m² | £27,000 – £40,000 | £45,000 – £65,000 | £55,000 – £80,000 |
| 40 m² | £33,000 – £48,000 | £55,000 – £78,000 | £65,000 – £85,000 |
These are estimates built from the per m² ranges above for a complete conversion without an en-suite, excluding VAT, not quotes for a specific house.
The four types of loft conversion
Each type suits a different roof, and the type sets the price band before anything else.
Rooflight (Velux) loft conversion cost
A rooflight conversion typically costs £22,000 to £40,000 nationally, or £25,000 to £45,000 in south Hampshire, and takes six to eight weeks on site.
The roof shape stays as it is. New floor joists go in, rooflight windows are set into the slope, and a staircase is added. It is the cheapest and quickest option and usually permitted development, but it only works if the loft already has enough head height. Most homes need around 2.2 metres from the top of the ceiling joists to the underside of the ridge.
Rear dormer loft conversion cost
A rear dormer typically costs £40,000 to £65,000 nationally, or £45,000 to £70,000 in south Hampshire, and takes eight to twelve weeks on site.
A box-shaped extension is built out from the rear roof slope, creating full-height walls and a flat roof. A full-width rear dormer is the most common conversion in the UK because it turns a sloping loft into a room you can stand up in almost everywhere, with space for an en-suite. On terraced streets it is often the only practical way to add a bedroom.
Hip-to-gable loft conversion cost
A hip-to-gable conversion typically costs £50,000 to £75,000 nationally, or £55,000 to £80,000 in south Hampshire, and takes ten to fourteen weeks on site.
On a semi or detached house with a hipped roof, the sloping side is built up into a vertical gable wall, extending the ridge and giving room for the staircase to come up in the middle of the house. It is usually combined with a rear dormer, which is why it costs more than either on its own.
Mansard loft conversion cost
A mansard typically costs £60,000 to £90,000 or more nationally, or £65,000 to £95,000 in south Hampshire, and takes ten to fourteen weeks on site.
The rear roof slope, and sometimes the front, is rebuilt almost vertically with a flat roof on top, effectively adding a full storey. It gives the most space and suits terraces in towns, but it nearly always needs full planning permission and takes longest to build.
What does a loft conversion cost, line by line?
The structure, meaning the steels, floor joists and dormer shell, is the largest cost at £12,000 to £35,000, followed by an en-suite at £6,000 to £15,000 if you add one.
| Item | Typical cost | Notes |
|---|---|---|
| Scaffolding | £1,500 – £4,000 | Depends on height, access and how long it stays up |
| Structure: steels, floor joists, dormer shell | £12,000 – £35,000 | The biggest single line on most quotes |
| Roof windows | £800 – £2,000 each, fitted | More for conservation-style or electric units |
| Staircase | £2,000 – £5,000 | Bespoke oak or a curved stair costs more |
| Insulation, plasterboard and plastering | £4,000 – £8,000 | Roof insulation to current Part L standards |
| Electrics and heating | £3,000 – £6,000 | New circuits, lighting, radiators or underfloor heating |
| Fire safety upgrades | £1,000 – £3,000 | Fire doors on the stairway, mains-wired smoke alarms |
| En-suite shower room | £6,000 – £15,000 | Depends on size, finish and the waste route |
| Eaves storage and fitted joinery | £2,000 – £6,000 | Built-ins make the most of the low spaces |
| Flooring and decoration | £2,000 – £4,500 |
Loft conversion cost by house type
The same type of conversion costs different amounts on different houses, mainly because of the roof shape, the party walls and where the stairs can go.
| House type | Most common conversion | Typical cost (south Hampshire, ex VAT) | Notes |
|---|---|---|---|
| Victorian or Edwardian terrace | Rear dormer, sometimes with an L-shaped dormer over the back addition | £45,000 – £70,000 | Party wall notices on both sides; tall roofs often convert well |
| 1930s semi with a hipped roof | Hip-to-gable plus rear dormer | £55,000 – £80,000 | Usually permitted development; one party wall |
| Detached house | Rear dormer or rooflight | £40,000 – £75,000 | No party walls; larger floor area |
| Bungalow | Dormer or hip-to-gable | £40,000 – £70,000 | Stair position is the main design question |
| 1960s onwards, trussed roof | Rear dormer with new steel supports | £50,000 – £75,000 | Trusses need an engineered replacement structure |
What fees do you pay on top of the build?
Drawings, structural calculations, building control and a Lawful Development Certificate typically add about £3,300 to £7,200 before any party wall costs.
- Architect or architectural technician drawings: £1,500 to £3,500
- Structural engineer calculations: £800 to £2,000
- Building control: £700 to £1,400, depending on the council or approved inspector
- Lawful Development Certificate (if permitted development): £274
- Householder planning application (if needed): £548
- Party wall surveyors (if a neighbour does not consent): roughly £700 to £3,500 per neighbour; see our guide to party wall agreements
Do you need planning permission for a loft conversion?
Many loft conversions are permitted development. In England the main limits, set out in the Planning Portal loft conversion guide, are:
- Volume: up to 40 cubic metres of additional roof space on a terraced house, 50 cubic metres on a semi-detached or detached house. A hip-to-gable counts towards this.
- No forward extension: nothing can extend beyond the roof slope facing the road.
- Height: nothing higher than the existing ridge.
- Materials: similar in appearance to the existing house.
- Side windows: obscure-glazed, and fixed shut below 1.7 metres from the floor.
- No balconies, verandas or raised platforms. A Juliet balcony is generally acceptable.
- Dormer set-back: where possible, at least 20 centimetres back from the eaves.
Roof extensions are not permitted development in conservation areas, National Parks, National Landscapes or World Heritage Sites, so a loft conversion inside the New Forest National Park needs planning permission, and some councils remove the rights elsewhere with Article 4 directions.
Bats and their roosts are protected by law. If bats use your loft, you need a bat survey before work starts and may need a licence from Natural England.
Mansards and front dormers almost always need full planning permission. Even when no application is needed, a Lawful Development Certificate is worth getting because a buyer's solicitor will ask for proof.
What building regulations apply to a loft conversion?
Every loft conversion needs building regulations approval, whether or not it needs planning permission. The main areas building control checks:
- Structure: floor joists, steel beams and the dormer, designed by a structural engineer.
- Fire safety (Approved Document B): in a typical two-storey house, the new floor needs a protected escape route down the stairs to the front door, fire doors to the habitable rooms off the stairway and mains-wired interlinked smoke alarms. Escape windows on their own are no longer accepted for a new storey this high.
- Stairs (Approved Document K): headroom of at least 2 metres, reduced to 1.9 metres at the centre of the stair for loft conversions, with a maximum pitch of 42 degrees.
- Insulation (Approved Document L) and ventilation (Approved Document F): to current standards, with ventilation to stop condensation in the roof.
- Sound (Approved Document E): insulation in the new floor between the loft and the bedrooms below.
- Electrics (Part P): new circuits must be installed by a registered electrician or signed off by building control.
At the end you receive a completion certificate. Keep it safe: you will need it when you sell.
Worked example: a rear dormer on a Southampton terrace
An illustrative estimate, not a past job, for a typical project on a Victorian or Edwardian terrace in Southampton or Portsmouth: a full-width rear dormer adding a double bedroom with a small en-suite, about 22 m² of new floor space.
| Item | Cost (ex VAT) |
|---|---|
| Scaffolding | £2,500 |
| Structure, steels and dormer shell | £26,000 |
| Two rooflights to the front slope | £2,800 |
| Staircase | £3,800 |
| Insulation, boarding and plastering | £6,200 |
| Electrics and heating | £4,200 |
| Fire doors and alarms | £1,800 |
| En-suite shower room | £8,500 |
| Eaves storage and joinery | £3,000 |
| Flooring and decoration | £2,700 |
| Fees: drawings, engineer, building control, party wall | £5,500 |
| Total | £67,000 |
That sits towards the top of the south Hampshire range for a rear dormer, because it includes an en-suite and fitted storage. Take out the en-suite and the figure falls to around £58,500; add a hip-to-gable on a semi and it rises towards £80,000.
The loft conversion process, step by step
- 1
Survey and feasibilityWeek 1
Measure the head height, check the roof structure, the stair position and the waste route for an en-suite, and agree a budget range.
- 2
Design and structural calculationsWeeks 2 to 6
Drawings for the layout, dormer and staircase, and a structural engineer’s design for the new floor and steels.
- 3
Approvals and party wall noticesWeeks 4 to 12
A Lawful Development Certificate or planning application, a building regulations application, and notices to neighbours where steels bear on a shared wall.
- 4
Scaffold and structureOn site, weeks 1 to 4
Scaffolding goes up, the new floor joists and steels go in, and the dormer or gable is built and made weathertight.
- 5
First fix and insulationWeeks 4 to 7
Electrics, plumbing and heating are run, then the roof is insulated, boarded and plastered.
- 6
Staircase and opening upWeeks 7 to 9
The ceiling below is opened, the new stair is fitted and the protected escape route and fire doors are completed.
- 7
Second fix, finishes and sign-offWeeks 9 to 12
En-suite, joinery, decoration and flooring, then the building control inspection and completion certificate.
How long does a loft conversion take?
Allow six to twelve weeks before work starts for drawings, structural calculations, approvals and party wall notices. On site, a rooflight conversion takes six to eight weeks, a rear dormer eight to twelve, and a hip-to-gable or mansard ten to fourteen. Most of the work happens from the scaffolding, and the ceiling below is only opened up for the staircase towards the end, so most families stay at home throughout.
What head height do you need for a loft conversion?
You need around 2.2 metres from the top of the ceiling joists to the underside of the ridge for most conversions to work without major roof changes.
Before you get quotes, check three things:
- Head height. Measure from the top of the ceiling joists to the underside of the ridge. Around 2.2 metres or more makes most conversions straightforward. Less than that usually means a dormer or a lowered ceiling below.
- Roof structure. A traditional cut roof with rafters and purlins is simpler to open up than a trussed roof.
- Staircase space. The new stair needs to land on the floor below, ideally over the existing stair, without taking too much from a bedroom.
How to keep the cost down
The biggest savings come from design decisions made before the quote, not from cheaper materials.
- Keep the stair over the existing stair. It saves a bedroom and simplifies the fire escape route.
- Put the en-suite over the bathroom below. A short waste run to the soil stack avoids pumps and boxing.
- Choose a rear dormer over a mansard where planning allows. It is quicker and cheaper for similar space.
- Decide on finishes before the quote. Changes once work starts cost more than getting them right on paper.
Mistakes that push the cost up
Most overspends on loft conversions come from five avoidable mistakes.
- Not checking the head height first. A loft that is a few centimetres short can mean lowering ceilings below or building a dormer you had not budgeted for.
- Leaving the staircase until last. The stair decides whether you lose part of a bedroom below, and it shapes the fire escape route.
- Ignoring the waste route. An en-suite far from the soil stack can need a pump, boxing-in or a raised floor.
- Serving party wall notices late. Two months' notice is the minimum for steels in a shared wall, and surveyors can add weeks if a neighbour dissents.
- Comparing quotes that are not like for like. Check whether each quote includes scaffolding, the staircase, fire doors, the en-suite, decoration and fees.
Questions to ask a loft conversion company
Ask these six questions before you compare quotes or sign anything.
- Is the quote broken down line by line, and what exactly is excluded?
- Who designs the structure, and will you see the engineer's calculations?
- Who handles the Lawful Development Certificate or planning application, building control and party wall notices?
- Which trades are your own, and which are subcontracted?
- What happens if you find something unexpected once the roof is opened up?
- Can you show a completion certificate from a recent loft conversion?
Loft conversions in Southampton and Portsmouth
Across Southampton and Portsmouth, the rear dormer is the most common conversion on Victorian and Edwardian terraces, and hip-to-gable conversions are common on 1930s semis in areas such as Shirley, Bitterne, Cosham and Drayton. Many of these are permitted development, but homes in conservation areas need full planning permission, and almost every terrace or semi needs party wall notices.
Our loft conversion service in Southampton covers the design, approvals and build with one team, including in Portsmouth, Fareham and Eastleigh, and if you are weighing a loft against building out, our guide to house extension costs uses the same pricing basis.
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We survey your loft, check the head height, roof structure and stair position, and give you a detailed written quote broken down line by line, with VAT stated clearly. Free loft survey, no obligation.
Written by
The Pure Renovation & Design team
A Southampton design-and-build company whose team has 18 years of hands-on experience in extensions, loft conversions, full renovations, bathrooms and joinery across Hampshire. Prices are cross-checked against published UK trade cost guides and official fee schedules, and reviewed whenever prices or rules change.
Last reviewed 13 October 2026 · About the team
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